A bar coater is employed in coating of paper, particularly in cases where the possibility exists that the coating blade in a blade coater will produce streaks in the paper surface. In order to avoid this problem, attempts have been made to prevent such streaks using a coating bar. As a rule, the coating bar is rotated in the direction opposite to the direction of running of the web, at a rate of from about 10 to about 600 revolutions per minute. Coating bars are provided with suitable drive gears to rotate the bar, and in wide machines the bars are usually provided with drives at each end of the bar to avoid torsional vibrations.
When a bar coater is used, the coating process itself can be arranged, e.g., so that the coating agent is scraped off the web surface by means of the coating bar. A bar coater may also be constructed as a so-called short-dwell unit, wherein the coating agent is introduced into a coating-agent chamber. A coating agent chamber is defined by a front wall, the coating bar, and by the base to be coated itself, which base may be the face of a counter roll, the paper web, or equivalent.
The coating bar is mounted such that it is able to revolve in a cradle made of a suitable material, such as polyurethane. Normally, the bar is supported in the cradle over its entire length. A groove for water is usually provided in the cradle, in connection with the bar. The water circulates in the groove in order to lubricate, to cleanse and to cool the coating bar.
Traditionally, a hard-chromium plated wire bar has been used. For example, the bar doctor in the SYM-SIZER size press (trademark of Valmet Paper Machinery, Inc.), a size press used for surface sizing and coating of paper operated by the principle of short-dwell coating, has traditionally been a bar around which a stainless-steel wire has been wound. Hereupon the bar has been hard-chromium plated to improve its resistance to wear. The wound wire forms regular slots in the bar surface, by means of which slots the quantity of size to be applied to the roll face can be regulated. The size of the slots and, consequently, the quantity of size can be regulated by using different wire diameters.
Drawbacks of such a wire bar include short service life, tendency of the wire to be broken and thereby to enter into the nip, with resulting damage to the roll coating and a standstill. Further problems include poor wear resistance of the bar, as well as unsuitability of the bar for thermal and thermo-chemical coating processes, because the wire may be broken during the process. Further difficulties arise in the quality of the coating process, because the coating does not become uniform with long bars and does not adhere properly.
A bar doctor composed of ceramic bushings is also known in the art, by whose means attempts have been made to solve the above problem of wear resistance. The success of such attempts has been unsatisfactory in practice. Grooved bars having a hard-chromium plating on their surface have also been employed.